Cable Theory and Extracellular Recording
In the previous chapters, the cell membrane was assumed to be isopotential; i.e., all points within the cell were at a value V m which changed only with time. However, experimental precautions have to be taken to ensure that this assumption is valid. In voltage clamping a squid axon, a long thin electrode is passed down the long axis of the fiber so that current is passed uniformly across the membrane and the membrane is therefore held at the same potential all along its length. In work on Purkinje fibers, the preparations are generally cut to a short length and then tied off or allowed to seal at the ends. The short length again ensures that the voltage is relatively uniform throughout. In studies on myelinated nerve, the nodal membrane, where appreciable ionic currents flow, is so small that it again is isopotential. Finally, in work on other tissues, artificial nodes are often created by isolating small gaps of membrane with a solution of sucrose. To voltage clamp a membrane using sucrose gaps. three gaps are created. The first is depolarized by applying a potassium solution of the same concentration as found internally. The voltage in the second is then measured with respect to the first and current is passed through the third node to change this potential as desired.
KeywordsNerve Fiber Conduction Velocity Fiber Diameter Myelinated Fiber Internodal Length
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